WO 94/01708 discloses a magnetic valve of this type. According to the embodiment shown, the said valve comprises a top housing part in the form of a thin-walled sleeve closed on its end and a bottom housing part with valve seat and passages for the inflow and outflow of a working medium. The bottom housing part is sealingly pressed into a valve accommodation, and the top housing part is encompassed by a magnetic coil. For connecting the two housing parts, a magnetic core is used which is fitted with its bottom cylindrical area into the bottom housing part and with its top cylindrical area of smaller diameter is fitted into the top housing part. The transition area has a conical design. The top housing part with its conically enlarged bottom end abuts on the transition area. The magnetic core and the top housing part are fastened above the transition area in the bottom housing part by displacement of plastic material. A tappet is guided in the magnetic core which cooperates with the valve seat and is connected to a longitudinally slidable armature that is arranged above the magnetic core in the top housing part. The armature is urged against the stop formed by the upper end of the top housing part by way of a resetting spring which is supported in the magnetic core and acts in the opening direction of the valve. When the magnetic coil is energized, a magnetic field develops and causes the armature to urge the tappet against the valve seat in overcoming the resistance of the resetting spring. Thus, the prior art magnetic valve is illustrated as a normally open valve, called NO valve in short.
The top housing part in this valve is made of a non-ferromagnetic material in order to prevent a magnetic short-circuit and permit operation of the valve with small losses only. This aspect is of special significance when, as is the case in anti-lock systems (ABS) or traction slip control systems (TCS) in automotive vehicles, there is the requirement of high closing pressures, on the one hand, and small overall dimensions, on the other hand. It is a general objective in these and other cases of application to minimize the magnetic losses to the greatest possible extent in order to achieve a compromise between contrasting requirements which is also reasonable under economical aspects.
Because the armature must be movable axially for functional reasons, a radial air gap is exactly as necessary as an axial residual air gap between the magnetic core and the armature in order to avoid a sticking connection. Also, the wall thickness of the top housing part which is not ferromagnetic must be considered as a loss-involving 'air gap'. More specifically, the magnetic losses which are produced by the sum of the air gaps in the prior art magnetic valve adopt a value which cannot be reduced any further, not even if highly narrow manufacturing tolerances are complied with.
The prior art magnetic valve suffers from the additional shortcoming that its adjustability is very intricate. During the assembly, a thin washer which corresponds to the residual air gap desired is interposed between the armature and the magnetic core. Thereafter, the top housing part with its contents is inserted into the bottom housing part until the magnetic core bears against the bottom housing part in an axial direction. The tappet which is initially inserted into the armature only in a light press fit and is supported on the valve seat is displaced upwardly relative to the armature. Subsequently, the parts are dismantled again, and the tappet is fixed in position in the armature by caulking.
In the second adjustment step, a washer which corresponds to the desired air gap in the deenergized condition, hence, which corresponds to the residual air gap plus working stroke desired is inserted between the armature and the magnetic core. All parts are assembled again, and the top housing part is urged into the bottom housing part until the magnetic core bears against the stop. In this arrangement, the armature is shortened plastically in its top weakened area by the size desired. Thereafter, the assembly must be dismantled and the washer removed before the parts can be reassembled and, as described hereinabove, coupled to each other in a third step.
The generic German utility model application No. 8522724.2 discloses a magnetic valve which includes a first cylindrical housing part, made of a ferromagnetic material, to form the magnetic core, and a second sleeve-type housing part, made of a ferromagnetic material, to accommodate a magnetic armature and a valve seat member which has a plurality of pressure fluid ports that are separated hydraulically by a tappet fitted to the magnetic armature in the initial position of the valve. Another housing portion which is made of a non-magnetic material is provided between the two housing parts. The above-mentioned separate housing parts are joined in the area of the non-magnetic material.
An object of the present invention is to improve upon a magnetic valve of the type mentioned hereinabove so that the magnetic losses in the working range of the valve can be reduced further, on the one hand, and that the valve adjustment can be simplified, on the other hand.
Surprisingly, it has shown that the losses due to magnetic short-circuits are reduced when conventional ferromagnetic automatic steel is used compared to a magnetic valve of the prior art type which is comparable in its overall size. This is due to the fact that, on the one hand, at least the losses caused by the radial air gap can be reduced and, on the other hand, magnetic saturation occurs very quickly in the area of the sleeve due to the small wall thickness. Even at operating pressures of 200 to 350 bar and in consideration of usual safety margins, the load-bearing sleeve cross-section can be reduced so that the short-circuit losses in the one-part housing are lower than in the prior art two-part housing with a top sleeve-type part made of a non-ferromagnetic material. Besides, material selection and a specific heat treatment offer further possibilities to shift the necessary compromise in designing the sleeve cross-section so that the magnetic short-circuit losses are further reduced.
Another advantage of the one-part housing design is that it obviates the need for adapting and joining steps and that it reduces the number of manufacturing tolerances to be considered. This becomes apparent especially in the adjustment of the magnetic valve, wherein due to the large number of parts cooperating in magnetic valves for an operating pressure of e.g. 200 to 220 bar, the operating pressure may vary between 200 and 350 bar without adjustment.